Why are some deserts covered with sand dunes while others are rocky?

A desert is defined mainly by low precipitation, not by sand. In some places, nearby sandstone or river deposits provide abundant sand that wind can shape into dunes, such as parts of the Sahara and the Namib. In other regions, the wind has blown away fine grains over long periods, leaving exposed bedrock, gravel plains, or rocky surfaces known as regs and hamadas. Vegetation is sparse, so there is little to trap or stabilize sediment. Whether dunes dominate depends on the balance between sediment supply and wind transport.